Method for determining the oxygen demand of an aqueous solution for a purification process

ABSTRACT

Method of determining the oxygen demand, TOD or COD, of an aqueous solution, in particular of waste water, for a clarification process, wherein a sample of the aqueous solution is decomposed by combustion, such that the combustion is carried out without the presence of a catalyst, at a temperature above 1150° C., in particular at 1200° C.

The invention relates to a method of determining the oxygen demand of anaqueous solution, in particular for a purification process.

A known method of determining the total content of organic carbon (TOC)in aqueous solutions, in particular waste water or also fresh water, isto subject the solution to high heat in a furnace and send thecombustion gas to suitable detectors in order to establish the presenceof compounds, the detection of which allows a conclusion to be drawnabout the amount of organic carbon in the aqueous solution. Suchcombustion procedures are customarily carried out in the temperaturerange between about 600 and 850° C., maximally up to 950° C. Thefurnaces used for this purpose are operated, as a rule, with a mainsvoltage of 220 V and employ Ta wires as heating elements. Combustionoccurs in the presence of a suitable catalyst and is therefore alsocalled thermal-catalytic release.

The patent DE 44 12 778 C1 discloses a method of analysing aparticle-containing aqueous sample, in particular for determining itsorganic carbon content, in which the sample is sprayed into a firstcombustion chamber which is then heated to about 1000° C. by anassociated heating device, and the sample is thereby evaporated andcombusted. After combustion is complete, the heating device in the firstcombustion chamber is turned off and the chamber is cooled down, and thecombustion gases are passed through a catalyst in a second, horizontallyoriented combustion chamber, where they are subjected to heat treatmentin a range between 800° C. and 950° C. The combustion chamber in thisarrangement is formed by an L-shaped quartz-glass tube. This secondarycombustion chamber is filled with an oxidation catalyst, for examplecopper oxide.

Another method of determining TOC is known from EP 0 887 643 A1. In thismethod the sample is first raised from an initial temperature, below thesimmering temperature of the water, to an evaporation temperature; thenin a second step it is brought up to a considerably higher combustiontemperature, preferably in the range between 800 and 1000° C. The use ofa catalyst is not specified in this document; however, that one isneeded is evident to the expert from the selected temperature range.

The document DE 199 23 139 A1 discloses a method and an apparatus fordecomposing an aqueous solution in order to determine its carbon contentin which the procedure includes a catalyst-free combustion at atemperature above 1000° C., in particular above 1200° C.

Another important quantity with respect to characterising thebiochemical and/or chemical quality of water for processing purposes—inparticular of sewage in preparation for clarification processes—is theamount of oxygen that will be required. This is in practicecharacterised by several quantities, of which the biochemical oxygendemand (BOD_(n)) has been used in practice for the longest time. Justlike the so-called chemical oxygen demand (COD), which was not adoptedfor sewage analysis until considerably later, this parameter isdetermined by means of a complicated, several-stage biochemical orchemical decomposition procedure. A short-term determination isdecidedly difficult, the measurement results are poorly reproducible andthe microorganisms are vulnerable to toxic matrix components, pH shiftsand the accumulation of inhibitory metabolic products. In contrast, thestandardised methods of COD determination provide excellentlyreproducible results, but cannot be well automated and make severedemands regarding protection of the workers.

A parameter for quantifying the organic load of waste water that isadvantageous from this point of view is the total oxygen demand (TOD),which is measured by a procedure including thermal oxidation bycombustion of the sample in a high-temperature reactor. This involvesdetection of not only the organic substances contained in the sample butalso, in part, other organic compounds. In many investigations a goodcorrelation has been found between COD and TOD, so that recently moreconsideration has been given to replacing the quantity COD by thequantity TOD as a quality parameter for water/sewage.

In the paper by W.-J. Becker “Zur Bestimmung des totalenSauerstoff-Bedarfs (TOD)”, X. f. Wasser- und Abwasser-Forschung, May 12,1979, 196 the procedure for TOD measurement is described in detail, anda survey of the main parameters of the most important commerciallyavailable COD and TOD waste-water analysis devices is given. From thissurvey it can be seen that the oxidation temperatures are mostly 900°C., in some cases also 850° C. The author of the publications mademeasurements at 1100° C. in a horizontal tube furnace.

The document JP-B-977-26111 describes a combined TOC and TOD measurementin which the sample is treated in a combustion chamber at 500° C.

During practical employment of the known catalytic thermal decompositionprocedures, i.e. in the routine operation of water-treatment plants,sewage works and the like, organizational problems have arisen regardingthe manipulation of the catalysts employed, which can be damaged bycertain sample components, in particular metal ions or complexes, andhence must be replaced periodically. Failure to carry out suchreplacements can, in some circumstances, cause erroneous measurementsand hence mismanagement of the processes controlled by the results ofthese measurements.

It is the objective of the invention to disclose a method of thisgeneric kind that is simpler to put into operation.

This objective is achieved by a method with the characteristics given inClaim 1.

The invention, accordingly, includes the essential idea of departingfrom the thermal-catalytic manner of decomposition and performing thisoperation entirely thermally, without using a catalyst. It furtherincludes the idea that for this purpose the combustion temperature isincreased, to values above 1150° C. and specifically to a value ofapproximately 1200° C.

Elimination of the catalyst required for the known methods isadvantageous not only with regard to costs—which vary depending on thenature of the catalyst—but primarily in that it produces the desiredfacilitation of the procedural organization, because there is no longerany need to keep a store of appropriate catalyst material available orto schedule and execute a periodic renewal of such material, and hence amore reliabile operation of the plant once it is no longer possible tomismanage such material. These advantages far outweigh the disadvantageof a slightly higher energy consumption during operation of the plant,owing to the higher combustion temperature.

It is particularly advantageous to carry out the method in an elongated,substantially vertically oriented reaction chamber. This offers theespecially advantageous opportunity to remove salts contained in thesample at the lower end of the reaction chamber and thus prevent“salinification” of the reaction chamber. This saves a considerableamount of maintenance work and, of course, the associated costs.

1. Method of determining the oxygen demand, TOD or COD, of an aqueous solution, in particular of waste water, in particular for a purification process, wherein a sample of the aqueous solution is decomposed by combustion, characterized in that the combustion is carried out without the presence of a catalyst in an elongated and substantially vertically oriented reaction chamber, to the upper region of which the aqueous solution is conducted, at a temperature above 1150° C., in particular at 1200° C.
 2. Method according to claim 1, characterized in that salts contained in the sample are removed at the lower end of the reaction chamber. 